@Caramon77; I'm not sure if I see the point of randomly trying codes. You can see ALL the known f/w versions in the
thread Direct Links to Kobo Firmware.
The only reason people are discussing this link
https://api.kobobooks.com/1.0/Upgrad...%1/%2/%3/%4/%5 (and the variant YOU tried) was looking for a means to speed up the determination of the availability of upgrades.